Contemporary heat pumps include a refrigeration circuit with a compressor and outdoor and indoor heat exchanger coils controlled to function alternately as an evaporator and a condenser in response to a thermostat controlled reversing valve calling for refrigerant flow direction for either heating or cooling cycles. During cooling cycles the indoor coil functions as an evaporator absorbing surplus heat from the indoor air, and the outdoor coil functions as a condenser rejecting that heat into the outdoor air.
During heating cycles the outdoor coil functions as an evaporator absorbing heat from the outdoor air, and the indoor coil functions as a condenser rejecting that heat to the indoor air for comfort heating. During the time outdoor temperatures are around 40 degrees, and colder, moisture from the outdoor air is collected onto the outdoor coil fins in the form of frost. The frost accumulates progressively in thickness on the fin surfaces thereby reducing heat transfer by blocking air flow therethrough, and by its insulating effect on the fin surfaces.
The frost accumulation is periodically removed by temporarily operating the heat pump in a cooling cycle wherein hot refrigerant vapor from the indoor coil is pumped through the compressor to the outdoor coil to heat it for frost removal. A defrost cycle is functionally a temporary cooling cycle. It is common practice to initiate defrost cycles by automatic means responsive to the thickness of frost accumulation, or by an interval timer, and to terminate the cycles by a thermostat sensing temperature rise of the coil, or its condensate, indicating completion of frost removal.
Each heat pump coil is usually provided with its own expansion device operative during the time the coil is serving as an evaporator. The device serving the outdoor coil in heating cycles provides for metering liquid refrigerant to efficiently meet the circumstances of evaporation during a range of cold outdoor winter temperatures. For example, at a winter ambient of 25.degree. F. the evaporating pressure in the outdoor coil would be approximately 35 psi, and the condensing pressure in the indoor coil 195 psi, establishing a pressure difference of 160 psi.
The expansion device serving the indoor coil during summer cooling cycles is constructed and adjusted to meter liquid refrigerant to the indoor coil under condensing pressure in the outdoor coil of approximately 250 psi, while the evaporating pressure in the indoor coil would be in the range of 72 psi, establishing a pressure difference of 178 psi, at 85.degree. F. ambient.
When a defrost cycle is initiated by establishing a temporary cooling cycle under the above conditions, the 35 psi condensing pressure in the outdoor coil is the maximum pressure available for introducing liquid refrigerant to the indoor coil through its high resistance expansion device adapted to controlling under a pressure difference in the range of 178 psi. The compressor is usually required to reduce the pressure in the indoor coil into a vacuum to produce the pressure differential necessary for feeding the indoor coil. The high resistance of the indoor expansion device, constructed to control refrigerant flow under a pressure differential in the range of 150 to 200 psi, offers excessive flow restriction under the defrosting cycle pressure differential in the range of 35 to 40 psi.
The sudden pressure drop from 35 psi condensing pressure requires the compressor to pump a vacuum that extracts, condenses, and delivers all the liquid refrigerant in the system into the outdoor coil. Coincidentally the refrigerant dissolved in the lubricating oil in the compressor crankcase expands into gas causing the oil to become foam having the consistency of whipped cream, which is also pumped into the outdoor coil. The liquid refrigerant and oil mixture is collected and retained in the outdoor coil during defrost cycles.
At the termination of each defrost cycle a heating cycle is started immediately by the change over of the reversing valve connecting the outdoor coil to the suction port of the compressor. Sudden delivery of the liquid and oil mixture from the outdoor coil to the compressor crankcase would be damaging to the compressor mechanism. Exposure to this damage is eliminated in contemporary heat pumps by a trap type accumulator installed between the reversing valve and the compressor suction port for collecting the inevitable surge of liquid refrigerant and oil mixture, and gradually releasing it into the compressor crankcase at a metered rate harmless to the compressor mechanism.
It is obvious that the trap type accumulator is an essential and effective device to protect the compressor in heat pump systems having the defrosting process described herein. Therefore compressor manufacturers, in their Engineering Bulletins for customer instruction, have made mandatory the use of trap type accumulators to protect their compressors and thereby reduce the number of compressor replacements under their warranties, as follows:
The Copeland Corporation, in its Engineering Bulletin No. AE-1243-R2, dated July 15, 1975, entitled "System Design for Air-to-Air Heat Pumps", includes the design requirements following:
"Unless the defrost cycle is such that liquid floodback does not occur, a suction accumulator is considered mandatory on all split systems, and on any system 3 horsepower and larger in size."
The Tecumseh Products Company, currently the world's largest manufacturer of compressors, in its Engineering Policy on Heat Pumps No. EP-4, revised Aug. 1, 1974, states:
"Consequently it is mandatory that a properly sized accumulator, of adequate oil return design, be used between the reversing valve and the compressor suction fitting. The accumulator should be sized to hold seventy-five percent of the system charge. A heat exchange (liquid to suction line) located between the accumulator and the compressor suction fitting is recommended to further help flood back to the compressor."
In view of the traditional heat pump design, and the compressor manufacturer's mandates for use of trap type accumulators, compressor engineers have followed the conventional design described herein and have accepted as inherent and unavoidable the detrimental effects of the accumulator on the defrosting process. This universal acceptance confirms the accumulator's advantages outweigh its principal disadvantages listed following:
1. It precludes the use of maximum heat from the indoor coil for defrosting by intercepting, cooling, and condensing hot gas from the indoor coil required for optimum defrosting in the shortest time. PA1 2. It increases the time length of defrost cycles and thereby reduces the heat pump coefficient of performance. PA1 3. It retains trapped liquid refrigerant during heating cycles, thus reducing residual liquid volume required in the indoor coil to accelerate the following defrost cycle with hot flash gas. PA1 4. It limits minimum outdoor temperatures at which defrosting can be accomplished, thus requiring a cold weather lockout thermostat which precludes economical operation in cold climates. It imposes restart problems after cold weather lockout during abnormal cold weather in mild climates. PA1 5. It precludes the use of air-to-air heat pumps as the universal replacement for combustion heating in all climates. PA1 1. An accumulator connected in the refrigerant circuit of a conventional air-to-air heat pump so that it will control and meter liquid floodback from the outdoor coil harmless to the compressor during heating cycles, and will have no detrimental effect on defrosting cycles. PA1 2. A refrigerant circuit bypassing the accumulator during defrosting cycles to provide direct flow of hot gas from the reversing valve to the outdoor coil to accelerate the defrosting process. PA1 3. Check valves for providing single direction refrigerant flow from the outdoor coil through the accumulator to the reversing valve only only during heating cycles. PA1 4. Check valve means providing single direction refrigerant flow from the reversing valve directly to the outdoor coil during defrost cycles, bypassing the accumulator, to accelerate defrosting with hot gas from the indoor coil by eliminating accumulator interception thereof. PA1 5. To minimize the time length of defrost cycles.